What were the labels in the 2009 surveys, exactly? I am a libertarian socialist, and in the 2011 survey I voted “socialism” because the examples made clear that the American (capitalist) meaning of libertarianism was intended, but if the options had been simply labelled “socialism”, “libertarianism” etc. with no example I would have voted the latter. If there are many other libertarian socialists around, this might explain much of the difference between the 2009 and 2011 results.